Industrial truck with a battery and method for operating an industrial truck with a battery

ABSTRACT

The invention relates to an industrial truck with at least one drive battery and at least one apparatus (A) for determining the state of charge of the battery and to a method for operating such an industrial truck. The apparatus (A) for determining the state of charge of the battery includes a device (B) for determining the dynamic internal resistance and a device (C) for determining the internal cell voltage of the battery with the aid of the dynamic internal resistance.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to German Application No. 10 2006 017889.0, filed Apr. 13, 2006, which application is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an industrial truck with at least one battery.The invention likewise relates to a method for operating an industrialtruck with at least one battery.

2. Technical Considerations

Such industrial trucks are used for the in-house transportation ofgoods, preferably when using electrical traction and lifting driveswhere it is necessary to avoid damaging exhaust gas emissions, inparticular in internal spaces. In comparison with vehicles driven by aninternal combustion engine, however, reduced use times need to beaccepted since recharging or replacement of the battery is substantiallymore complex and more time-intensive than a tank-filling operation. Forthis reason, frequent charging or replacement operations should beavoided, which can be achieved in particular by optimized utilization ofthe energy supply stored in the battery. One of the preconditions forthis is knowledge of the present state of charge of the battery which isas precise as possible since, when the available charge isunderestimated, charging operations which are too frequent occur, while,when the state of charge is overestimated, the vehicle may failunexpectedly, which means a substantially more serious restriction ofthe use time and, owing to the deep discharge, results in a shortenedlife of the battery.

In order to determine the energy supply stored in the battery, variousmethods are known. One of these consists in determining the state ofcharge by measuring the open terminal voltage of the battery. These twovariables are approximately proportional to one another when the batteryis off load. When an industrial truck is operated, however, variousfactors occur which may result in a falsification of the result: since,on load, the battery voltage is reduced, the energy content available isunderestimated in the case of a state-of-charge display based on thevoltage, which results in an unnecessarily premature charging operation.

A further problem in all methods which are based on voltage measurementsis the occurrence of the so-called diffusion overvoltage, in which,owing to the loading of the battery, a concentration gradient occurs inthe electrolyte, which results in a reduction in the terminal voltage.This phenomenon occurs particularly markedly in the case of thelead-acid batteries usually used in industrial trucks. As adiffusion-controlled process, the level of the diffusion overvoltagechanges in the event of a change in the power drawn with a considerabletime delay, with the result that, in the case of state-of-chargedisplays which are based on the measured terminal voltage, the displayof the state of charge lags behind the actual available capacity. Ifthis is not taken into account, the energy supply on load isoverestimated and this may result in a damaging deep discharge of thebattery. When there is no load, the distribution is brought back tonormal again after a certain rest time, and the full capacity isavailable again. A simple state-of-charge display based on the batteryvoltage is therefore associated with many uncertainties and is thereforeonly suitable in certain circumstances for use in an industrial truck.

Greater accuracy in terms of the energy supply stored in the battery isobtained with methods in which, owing to the determination of theincoming or outgoing currents and integration of these measured values,the charge content of the battery is determined. In the event of abattery replacement, as is often carried out in practice in the case ofindustrial trucks, since the time required for this is substantiallyshorter than that required for charging the battery, however, the amountof usable charge contained in the new battery is not known. Thisdisadvantage can be circumvented in part by battery data being stored onthe battery or in a control device, but this is relatively complex.

Methods are also known which carry out both current and voltagemeasurements in order to determine the state of charge of the battery.For example, the capacity of the battery when inserted into the vehicleis determined from the measured open-circuit voltage and, when power isdrawn, by integrating the measured current the drawn charge andtherefore also the remaining residual capacity are determined. It isalso known from U.S. Pat. No. 4,333,149 to calculate a so-called dynamicinternal resistance, which has a relationship with respect to the stateof charge, from the voltage drop when the battery is on load and fromthe drawn current. However, this method requires precise knowledge onthe battery and does not take sufficient account of many boundaryconditions, such as line losses between electrodes and measuringdevices, for example. In order to obtain such knowledge, lengthylearning phases are required for each battery. In the case of anindustrial truck which is often operated with replaceable batteries,this complex adaptation also needs to be carried out for each battery,and the apparatus needs to be operated with the respectively appropriatebattery parameters. In the case of these methods, too, owing to thedependence on the measured voltage values, the occurrence of thediffusion overvoltage may result in a falsification of the informationon the state of charge.

The methods or apparatuses known for industrial trucks with at least onebattery in accordance with the known art for calculating and displayingthe electrical energy which can be drawn from the battery are thereforeeither very complex or only produce unreliable results, which eithersystematically over-or underestimate the true state, depending on themethod, or else deviate unpredictably depending on the dischargeconditions.

Disadvantageous factors here, in particular in addition to the complexdesign, are the slow response to the severely fluctuating operatingconditions of an industrial truck and the associated influences on theavailable battery capacity as well as the necessity to determine preciseinformation on the battery used using complex methods. That is to say,if incorrect battery parameters are used, in most methods this mayresult in serious erroneous estimations of the state of charge with theknown negative consequences.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an industrial truck with atleast one battery in which the residual capacity of the batteryavailable in different operating and loading conditions can be detectedin a reliable manner with little complexity for control tasks and/or fordisplay to the operator.

Furthermore, it is object of the invention to provide a method foroperating an industrial truck with at least one battery, with whichmethod the residual capacity of the battery available in differentoperating and loading conditions can be detected in a reliable mannerwith little complexity for control tasks and/or for display to theoperator. This object is achieved with respect to the industrial truckby virtue of the fact that the apparatus for determining the state ofcharge of the battery comprises means for determining the dynamicinternal resistance and means for determining the internal cell voltageof the battery with the aid of the dynamic internal resistance. In thiscase, the internal cell voltage is considered to be the potentialdifference between the terminals in a battery. The internal cell voltageis a good measure of the state of charge of the battery, but differsconsiderably from the voltage present at the terminals of the cell,which voltage can be measured directly. The dynamic internal resistancecan likewise be used as a parameter for calculating the state of charge,whereby the present determining methods in which the dynamic internalresistance is calculated from the quotient between the differencebetween the open terminal voltage of the battery in the rest state andthe voltage on load and the current drawn likewise result inbattery-dependent errors, which in particular result from the lineresistances not being taken into account. By the internal cell voltagebeing determined with the aid of the dynamic internal resistance, a veryprecise method for determining this voltage is provided. In comparisonwith methods in accordance with the known art which only use the voltagemeasured at the terminals for determining the internal cell voltage andcorrect this voltage on the basis of estimations and assumptions on thebattery, a substantially improved degree of accuracy is thereforeachieved.

Regarding the method, the object is achieved by virtue of the fact thatthe internal cell voltage of the battery is determined with the aid ofthe dynamic internal resistance of the battery, and the state of chargeof the battery is determined from the internal cell voltage of thebattery.

The other dependent claims describe advantageous embodiments of theinvention.

Further advantages and details of the invention will be explained inmore detail below with reference to the exemplary embodiment illustratedin the drawings. Identical parts have been identified by the samereference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an apparatus according to theinvention for determining the state of charge of the battery of anindustrial truck according to the invention, and

FIG. 2 shows a schematic of the design of an apparatus (3) fordetermining the battery cell voltage as part of an apparatus accordingto the invention for determining the state of charge of the battery ofan industrial truck according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of an apparatus A according to theinvention for determining the state of charge of the battery of anindustrial truck according to the invention to illustrate the sequenceof a method according to the invention. The apparatus A for determiningthe state of charge of the battery comprises means B (see FIG. 2) fordetermining the dynamic internal resistance and means C for determiningthe internal cell voltage of the battery with the aid of the dynamicinternal resistance.

In the apparatus (1) for determining the battery terminal voltage, thevoltage U present at the battery of the industrial truck is determinedand divided by the number of cells n provided in the battery (whichresults from the vehicle-specific rated voltage), and thus the voltagevalue u is determined which is present on average at each battery cell.However, this division can also take place in principle at another pointin the processing chain of the data, if this is necessary or expedient,for example for reasons of computational accuracy. Such solutions cannaturally be regarded as being equivalent.

In the apparatus (2) for determining the current drawn from the battery(not illustrated here), the total current drawn from the battery isdetected.

The apparatuses (1, 2) for determining the values for the voltage u andthe current i may be in the form of conventional measuring devices forthis purpose in accordance with the known art or else, in particular inthe case of the current, in the form of calculation apparatuses, whichcalculate the values from other data, such as are detected, for example,in the vehicle controller.

In an apparatus (3) for determining the battery cell voltage, theinternal cell voltage of the battery is determined from the current andvoltage values i, u, while, parallel to this, in an apparatus (4) fordetermining the diffusion overvoltage, the value for the diffusionovervoltage, which counteracts the cell voltage owing to a concentrationgradient in the electrolyte, is likewise determined from the current andvoltage values i, u. The manner in which the apparatus (3) fordetermining the battery cell voltage functions is explained in moredetail further below with reference to FIG. 2.

The diffusion overvoltage is in opposition to the battery cell voltageand, as the battery-specific variable, is essentially dependent on thepower drawn, the geometry of the battery and the composition of theelectrolyte. The apparatus (4) for determining the diffusion overvoltagecalculates the level of the diffusion overvoltage using empiricallydetermined average values from the input variables which makecalculation of the power drawn possible. The adjustment is selected suchthat overcompensation is avoided which would result in ageing effectsowing to the incorrectly calculated battery capacity. The apparatus (4)for determining the diffusion overvoltage in the present exemplaryembodiment comprises an attenuator, which calculates the level of thediffusion overvoltage as a function of the power drawn, and a low-passfilter with a PT1 response, which reproduces the time delay between thewithdrawal of power and the change in the diffusion overvoltage. Therelationship between the withdrawal of power and the diffusionovervoltage is in this case produced in the form of an empiricallydetermined equation in the attenuator, but a value assignment controlledby families of characteristics is also conceivable.

In the apparatus (5) for determining the internal open-circuit cellvoltage, the (opposite) values for the internal cell voltage and thediffusion overvoltage are added to one another and thus the internalcell voltage of the battery in the rest state, i.e. with no load, isdetermined. This characteristic variable is directly related to thestate of charge of the battery, which is determined in the apparatus (6)for determining the state of charge. The relationship between theinternal open-circuit cell voltage and the state of charge is stored inthe apparatus (6) in the form of an equation for calculating the stateof charge from the internal open-circuit cell voltage, but embodimentsare also conceivable in which a value assignment controlled by familiesof characteristics is carried out. In the apparatus (6) for determiningthe state of charge, the time profile of the internal open-circuit cellvoltage is also analyzed. If this changes within a predetermined timeinterval more severely than a predetermined limit value (i.e. if a kinkin the curve which represents the internal open-circuit cell voltageover time is established), this indicates the beginning of a damagingdeep discharge.

The value for the state of charge in the industrial truck illustrated isrepresented in a suitable display apparatus (8), with the result thatthe operator is always informed of the residual capacity available. Thedisplay is in the form of a percentage display of full battery capacity,but other embodiments in accordance with the known art are alsoconceivable, such as audible indications or a display of the residualrunning time, for example. Furthermore, a vehicle controller (7) is alsoconnected to the apparatus (6) for determining the state of charge,which vehicle controller controls the power of individual loads as afunction of the state of charge of the battery in order to ensure thatthe energy of the battery is used optimally and safety-relevantfunctions remain functional even when there is a low state of charge.The transmission of the battery state of charge to further deviceswithin or outside of the industrial truck is possible via a transmissionapparatus (9), for example by means of a radio link to a central controlpoint, which coordinates the use of various industrial trucks. If theonset of a deep discharge is established in the apparatus (6) fordetermining the state of charge, a corresponding signal is output to theconnected units (7, 8, 9) and suitable measures are introduced there inorder to avoid further discharge, i.e. for example a warning indicationis output to the operator, the vehicle is brought to a standstill or acentral control point is informed of the threat of failure of thevehicle.

FIG. 2 illustrates a schematic of the design of an apparatus (3) fordetermining the battery cell voltage as part of an apparatus accordingto the invention for determining the state of charge of the battery ofan industrial truck according to the invention. This configuration isused to illustrate the method according to the invention for operatingan industrial truck.

The apparatus (3) for determining the battery cell voltage comprises twodifferentiating apparatuses (10, 11), a division unit (12), a low-passfilter (13), a multiplication unit (14) and an addition unit (15).

In a first step, the current i is differentiated over time in thedifferentiating apparatus (10) and the voltage u is differentiated overtime in the second differentiating apparatus (11), and thus the changein these variables is determined as a function of time. In thedifferentiating apparatus (12), the quotient of the values di/dt anddu/dt determined in the differentiating apparatuses (10, 11) is formed.This value represents the dynamic internal resistance R_(i)(t) of thebattery and the lines connected to it. In contrast to methods inaccordance with the known art which calculate the internal resistanceonly from the quotient between the difference between the open terminalvoltage of the battery in the rest state and the voltage on load and thecurrent drawn, a more precise value is therefore achieved. The low-passfilter (13) stabilizes the value of R_(i)(t). The stabilized value ofR_(i)(t) is multiplied in the multiplication unit (14) by the current i,and thus the voltage dip U_(drop) which takes place owing to the dynamicloading is determined. In the addition unit (15), the voltage dipU_(drop) is added to the terminal voltage of the individual cell u andthus the internal cell voltage U_(cell) is determined, which is thenfurther-processed in the above-described manner in the apparatus (5) fordetermining the internal open-circuit cell voltage.

The apparatuses may be in the form of independent components orintegrated in a single circuit. Furthermore, complete or partialintegration into the circuits of a vehicle controller is conceivable, asis implementation of the calculation processes of the individualcomponents in a computer. The apparatus according to the invention andthe method according to the invention are particularly suited forapplication in industrial trucks in which a lead-acid battery is used asthe traction battery, in particular owing to the compensation for thediffusion overvoltage, which occurs to a particularly pronounced extentin such batteries. However, in principle an application is also possiblewhen using other batteries, such as nickel metal hydride, lithium ion orlithium ion polymer batteries.

Since, with the exception of the information on the diffusionovervoltage, no further battery parameters need to be known, complexbattery data storage systems or learning phases for adapting to the typeof battery used are not required. In the case of altered conditions, forexample intermediate charging on a charging device or owing to energyrecovery during operation, the arrangement shown responds more quicklyto the changes than apparatuses in accordance with the prior art. Owingto the short response time, for example, deep discharges are preventedwhich arise if the state of charge display indicates a value which istoo high and the industrial truck is therefore operated for longer thanappropriate for the actual state of charge.

1. An industrial truck, comprising: at least one drive battery; and atleast one apparatus for determining a state of charge of the battery,wherein the apparatus for determining the state of charge of the batterycomprises means for determining a dynamic internal resistance and meansfor determining an internal cell voltage of the battery with the aid ofthe dynamic internal resistance.
 2. The industrial truck according toclaim 1, wherein the means for determining the internal cell voltage andthe means for determining the dynamic internal resistance comprise atleast one apparatus for determining a terminal voltage of the batteryand at least one apparatus for determining a current drawn from thebattery.
 3. The industrial truck according to claim 1, wherein the meansfor determining the dynamic internal resistance comprise at least oneapparatus for determining a time profile of a change in the terminalvoltage of the battery and/or in the current drawn from the battery. 4.The industrial truck according to claim 2, wherein the means fordetermining the dynamic internal resistance comprise at least oneapparatus for determining a time profile of a change in the terminalvoltage of the battery and/or in the current drawn from the battery. 5.The industrial truck according to claim 1, wherein the apparatus fordetermining the state of charge of the battery comprises means forcompensating for a diffusion overvoltage.
 6. The industrial truckaccording to claim 2, wherein the apparatus for determining the state ofcharge of the battery comprises means for compensating for a diffusionovervoltage.
 7. The industrial truck according to claim 3, wherein theapparatus for determining the state of charge of the battery comprisesmeans for compensating for a diffusion overvoltage.
 8. The industrialtruck according to claim 1, further comprising means for detecting athreat of a deep discharge from the time profile of the internalopen-circuit cell voltage are provided.
 9. The industrial truckaccording to claim 2, further comprising means for detecting a threat ofa deep discharge from the time profile of the internal open-circuit cellvoltage are provided.
 10. The industrial truck according to claim 3,further comprising means for detecting a threat of a deep discharge fromthe time profile of the internal open-circuit cell voltage are provided.11. The industrial truck according to claim 4, further comprising meansfor detecting a threat of a deep discharge from the time profile of theinternal open-circuit cell voltage are provided.
 12. A method foroperating an industrial truck, comprising: determining an internal cellvoltage of a battery with the aid of a dynamic internal resistance ofthe battery, and determining a state of charge of the battery from theinternal cell voltage of the battery.
 13. The method for operating anindustrial truck according to claim 12, wherein the dynamic batteryinternal resistance is determined from a time profile of a terminalvoltage of the battery and of current drawn from the battery.
 14. Themethod for operating an industrial truck according to claim 12, whereinthe dynamic internal resistance is determined from a quotient of a timederivative of a terminal voltage of the battery and of a current drawnfrom the battery.
 15. The method for operating an industrial truckaccording to claim 13, wherein the dynamic internal resistance isdetermined from a quotient of a time derivative of a terminal voltage ofthe battery and of a current drawn from the battery.
 16. A method foroperating an industrial truck according to claim 12, wherein theinternal cell voltage is determined on the basis of multiplication of acurrent drawn from the battery and a dynamic internal resistance.
 17. Amethod for operating an industrial truck according to claim 13, whereinthe internal cell voltage is determined on the basis of multiplicationof a current drawn from the battery and a dynamic internal resistance.18. A method for operating an industrial truck according to claim 14,wherein the internal cell voltage is determined on the basis ofmultiplication of a current drawn from the battery and a dynamicinternal resistance.
 19. The method for operating an industrial truckaccording to claim 12, wherein an internal open-circuit cell voltage iscalculated from addition of an internal cell terminal voltage and adiffusion overvoltage.
 20. The method for operating an industrial truckaccording to claim 12, wherein the state of charge is determined from aninternal open-circuit cell voltage.